1. Field of the Invention
This invention concerns a method of producing reduced and purified metal from a metal-containing raw and at least partially oxidized material, and to a traveling hearth furnace for producing such reduced metal. More particularly, this invention relates to a novel technique of charging and stacking metal-containing material on a hearth that is moving in a furnace, for heating and reducing the metal-containing material by heating during movement of the hearth. This invention continuously produces reduced metal of high quality from a traveling hearth furnace, with unexpected economy and efficiency.
2. Description of the Related Art
Steels are generally produced either by a converter or an electric furnace. Electric furnace steels are produced by heating and melting scrap or reduced iron by using electrical energy, followed optionally by refining the same. However, since the supply of scrap has become scarce and the demand for high quality steels has increased more and more in recent years, reduced iron has been used more than scrap.
As one reduced iron process, the method of the traveling hearth furnace has been known. One example is shown in Japanese Patent Unexamined Publication Sho 63-108188. Iron ores and solid reducing materials are charged onto a hearth moving in a horizontal direction, and the iron ores are heated and reduced by radiant heat transfer from above, to produce reduced iron. As shown in FIG. 1 of that Japanese publication, charged raw material can be heated during horizontal movement of the hearth. The hearth is usually adapted to rotate as shown, and the hearth furnace is usually referred to as a rotary hearth furnace.
As shown in FIG. 1(a) of the drawings of this application, the rotary hearth furnace has an annular hearth body partitioned into a preheating zone 10a, a reducing zone 10b and a cooling zone 10d, located along the supply side to the discharge side of the furnace. An annular hearth 11 is supported in the furnace body so as to move rotationally.
As shown in FIG. 1(b) of the drawings, a raw material 2 comprising a mixture, for example, of iron ore and solid reducing material, is charged. Pellets having incorporated carbonaceous material are preferably used. The hearth 11 has refractory applied on the surface thereof, or granular refractory may be stacked. A burner 13 is disposed in an upper portion of the furnace body, and metal-containing oxides such as iron ores stacked on the hearth 11 are reduced by heating in the presence of the reducing material into reduced iron by using the burner 13 as a heat source.
In FIG. 1(a) of the drawings, the number 14 represents a feed device for charging the raw material onto the hearth, and 15 denotes a discharge device for the reduced product.
In the usual operation of a traveling hearth furnace, the atmospheric temperature in the furnace body 10 is preferably about 1300.degree. C. The reduced product after completion of the reducing treatment is cooled at the cooling zone 10d (FIG. 1(a)) on the rotating hearth 11, for preventing reoxidation and facilitating discharge from the furnace.
In the operation of the traveling hearth of Japanese Patent Unexamined Publication Sho 63-108188), in conducting the reducing reaction between the iron ore and the solid reducing material, improved productivity is intended to be obtained by decreasing the thickness of the raw material layer and increasing the moving speed of the hearth. However, serious problems arise, as will be detailed hereinafter.
The usual metal-containing materials, for example iron ores, contain a great amount of a gangue ingredient, although this varies depending on the place of production. Coal, coal char and coke, which are typical solid reducing materials, contain a substantial ash ingredient. Accordingly, if the reduced iron is produced only by a reduction reaction, it is inevitable that a great amount of gangue remains in the reduced iron product. Further, ashes adhere to the reducing material and contaminate the reduced iron.
If reduced iron containing a great amount of gangue and ash are then charged into an electric furnace, the amount of calcium oxide that controls the slag CaO/SiO ratio for dephosphorization and desulfurization is increased. This seriously increases the cost, as well as the amount of electric power used, along with increase of heat energy required for formation of slag.
Further, reduced iron obtained only by a reducing reaction usually contains a substantial number of pores, making the iron highly reoxidizable when stored in atmospheric air. This deteriorates the quality of the product and even suffers from the danger of fire caused by generation of heat upon reoxidation. Further, since the porous reduced iron has a low apparent density due to the presence of pores, it floats on slag when used in an electric furnace, sometimes making it difficult to achieve smooth melting and refining. In addition, if the size of the reduced iron product is too great, it takes a long time to melt it in the electric furnace, thereby slowing the productivity of the electric furnace. Accordingly, it is indispensable to decrease the size of the reduced iron.
Accordingly it has been demanded, in the operation of traveling hearth furnaces, to use iron ores of high quality with the gangue percentage as low as possible, and to use a reducing material having an ash content as low as possible. However, sources of pure iron ores or high quality coals are very scarce and expensive. In fact, materials of low quality have to be used whenever possible.
In view of the background as described above, there is a great need for effectively separating a metallic ingredient such as Fe from the usual gangue ingredient and recovering a metallic ingredient, and to do this in the operation of a traveling hearth furnace.